ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657
www.elsevier.com/locate/nima

A large area muon tracking detector for ultra-high energy
cosmic ray astrophysics—the GRAPES-3 experiment
Y. Hayashia, Y. Aikawaa, N.V. Gopalakrishnanb, S.K. Guptab,, N. Ikedaa,
N. Itoa, A. Jainb, A.V. Johnb, S. Karthikeyanb, S. Kawakamia, H. Kojimac,
T. Matsuyamaa, D.K. Mohantyb, P.K. Mohantyb, S.D. Morrisb, T. Nonakaa,
A. Oshimaa, B.S. Raob, K.C. Ravindranb, M. Sasanoa, K. Sivaprasadb,
B.V. Sreekantanb, H. Tanakaa, S.C. Tonwarb, K. Viswanathanb, T. YoshikoshiaaGraduate School of Science, Osaka City University, Osaka 558-8585, Japanb
Tata Institute of Fundamental Research, Mumbai 400 005, Indiac
Nagoya Women’s University, Nagoya 467-8610, Japan

Received 1 November 2004; received in revised form 26 January 2005; accepted 9 February 2005
Available online 15 April 2005
(The GRAPES-3 Collaboration)

Abstract
Certain aspects of ultra-high energy (UHE) cosmic ray astrophysics require correlated studies on the electron andmuon components of air showers, namely, the search for cosmic ray sources through g-ray astronomy and studies on
the variation of the nuclear composition of primary cosmic rays with energy. While studies on the electron component
provide basic information about the arrival direction and energy, it is the muon component that plays a crucial role indistinguishing primary g-rays from charged cosmic ray particles and in determining the composition.
A large area (560 m2 ), tracking muon detector of the GRAPES-3 experiment, operating at Ooty in southern India,
has been designed for detailed studies on both of these aspects of UHE cosmic ray astrophysics. We present here thedetails of the muon detector, associated electronics and the data acquisition system. A brief discussion of the potentialof the muon detector is presented through simulation studies.
r 2005 Elsevier B.V. All rights reserved.
PACS: 98.70.Sa; 96.40.Pq; 96.40.De; 98.70.Rz; 95.55.Vj; 13.85.Tp
Keywords: Cosmic rays; Extensive air showers; Composition; Energy spectra and interactions; Gamma-ray sources; Cosmic ray
detectors; Cosmic ray interactions

Corresponding author. Tel.: +91 22 2280 4545; fax: +91 22 2280 4610.

E-mail address: gupta@grapes.tifr.res.in (S.K. Gupta).
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2005.02.020

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Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657

1. Introduction
Cosmic rays span an enormous range of energy
from 108 eV to beyond 1020 eV with an energy
spectrum that is essentially a continuous powerlaw. The high interest in cosmic ray astrophysics[1] continues to be driven by the fact that it isalmost impossible to understand all the available
information on high energy cosmic rays throughknown acceleration processes and nature ofastrophysical sources. Owing to the deﬂection of
charged cosmic rays by the randomly oriented
interstellar magnetic ﬁelds, the cosmic ray ﬂux
incident on the Earth is highly isotropic and loses
all the information about the sources and theirdirections. Acceleration [2,3] of cosmic rays has
been associated with supernovae for a long timeessentially due to their large energy output andbecause of the success of the phenomenon ofdiffusive shock acceleration [4], also known as theﬁrst-order Fermi acceleration mechanism, in accelerating particles up to energies 1015 eV (PeV).
The energy reach of this process is further
extended by invoking acceleration by shocks
propagating through stellar and galactic winds.
The expected nuclear composition of particles
accelerated through different variants of thismodel is somewhat different and new measurements [5] may provide clues for resolving themystery of the nature of the sources of ultra-highenergy (UHE) cosmic rays.
Observational studies during recent years havefocused on two aspects of UHE cosmic ray
astrophysics, namely, direct studies on the sourcesof g-rays and indirect studies on the nuclear
composition of cosmic rays. Although the importance of studies [6–8] on cosmic g-rays for very
high energy (VHE) cosmic ray astrophysics was
highlighted in the late 1940s and 50s, the ﬁeld of
VHE g-ray astronomy [9] advanced only in 1980s
following the breakthrough provided by thedevelopment of the imaging technique [10,11].
This was essentially due to an efﬁcient rejection of
the large background of charged cosmic rays.
Similarly, at the UHE (1014 –1016 eV), anotherbreakthrough has been sought by the use of themuon component [12,13] for identifying g-initiatedshowers in the presence of a large ﬂux of showers

due to charged cosmic rays. The Chicago Air
Shower Array (CASA) in association with theMichigan Muon Array (MIA) was the ﬁrst
experiment to use an array of large area muon
detectors to search [14] for muon-poor showers asa signal from potential UHE g-ray sources. The
EAS-TOP [15], the KASCADE [16] and the
GRAPES-2 [17] are the other recent experiments
using either a large area muon detector or an arrayof muon detectors with the capability to observemuon-poor showers. However, the GRAPES-3
experiment [18,19,21] with its single large area
(560 m2 ) muon detector is attempting to reach asigniﬁcantly higher level of sensitivity to g-initiatedshowers based on their small muon content,
particularly at sub-PeV energies, lower than thoseachieved by the CASA–MIA experiment. On theother hand, by using Čherenkov radiation in water
as the particle detection mechanism, the MILAGRO [22] experiment is attempting to achieve fullparticle detection sensitivity over a large area by
instrumenting a 8 m deep water pond of 80 
60 m2 area with two layers of photomultipliers.
The lower layer, located deeper in water, is used todetect the muons. However, reconstruction ofparticle (electron or muon) tracks in the MILAGRO experiment is easier for smaller showers in
the TeV (1012 eV) energy range and may be more
complicated for showers in the PeV range.
Direct measurements [5] on the energy spectraof individual nuclei present in primary cosmic ray
ﬂux have been possible with satellite and balloonborne detectors only up to a total energy t1 TeV
due to a steep fall in the ﬂux with increasing energy
and loss of accuracy in particle identiﬁcation andenergy measurement. The energy range has been
extended to a total energy 100 TeV by the use of
nuclear emulsion and/or X-ray ﬁlm as the detector
element in shallow stacks of absorber ﬂown
aboard balloon platforms in long-duration ﬂights[23,24]. These experiments have provided valuable
data on the ﬂux of groups of nuclei, for example,
the CNO group, the Medium Heavy ð12oZo20Þ
group and the Heavy ðZX20Þ group. Due tosevere practical constraints on the long duration
ﬂights with heavy payloads, studies in cosmic ray
astrophysics at energies \100 TeV have to necessarily rely on indirect measurement of air showers

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carried out at mountain altitude, sea level orunderground locations. Due to combined effects ofenergy dependence of: (i) secondary multiplicity inparticle interactions, (ii) interaction cross-sectionsof particles with air nuclei, (iii) decay life-time forkaons, pions, muons and (iv) nearly exponential
nature of the density proﬁle of the atmosphere, themuon component offers the highest sensitivity [17]for studies on the nuclear composition of primarycosmic ray ﬂux at PeV energies. However theelectrons, in an air shower being numericallydominant component, are used for obtaining
information on various shower parameters, suchas the arrival direction, the energy of the primaryparticle and also for generating a trigger.
The muon component has been studied in recent
experiments in two different ways. Using an arrayof muon detectors, the muon size is determined forindividual showers from an integration over thelateral distribution function as ﬁtted to theobserved muon densities, for example in the
CASA–MIA [25] and KASCADE [16] experiments. Using a single large area muon detector,
such as in the EAS-TOP experiment [15], the muon
size has been estimated for individual showers
from the lateral distribution function obtained
from a ﬁt to the data. On the other hand, the
GRAPES-2 [17] group has made a detailed studyof the muon multiplicity distribution in an attempt
to obtain the ﬂux of light nuclei (H+He+CNO
groups) and heavy nuclei (Si+Fe groups) incosmic rays almost independently. Pursuing thestrategy of the GRAPES-2 group, the GRAPES-3
collaboration has constructed a large, 560 m2 area,
muon detector to study the muon multiplicity
distribution for showers over a wide energy range.
The GRAPES-3 experiment is located at Ooty
(11:4 N latitude, 76:7 E longitude and 2200 m
altitude), a popular mountain resort town insouthern India. The general layout of the
GRAPES-3 muon detector is presented in Section
2 including the details of main detector element,
namely the proportional counter (Section 2.1), thesignal processing electronics and the data acquisition system (Section 2.2), and monitoring of the
PRCs (Section 2.3). In Section 3 some of theobservations showing the performance of thedetector and its sensitivity for various kinds of

645

measurements are presented. In Section 4 theresults from Monte Carlo simulations which
demonstrate its potential for measurements on
the nuclear composition of primary cosmic rays
over PeV energy range are discussed. Finally, asummary is presented in Section 5.

2. The GRAPES-3 muon detector
Proportional counter (PRC) is the basic elementof the GRAPES-3 muon detector. The PRC is a
600 cm long, mild-steel, square pipe with a crosssectional area of 10 cm  10 cm and a wall thickness of 2.3 mm as shown in Fig. 1.
A muon detector module with a sensitive area of
35 m2 consists of a total of 232 PRCs arranged in 4
layers, with alternate layers placed in orthogonal
directions. Two successive layers of PRCs are
separated by 15 cm thick concrete, consisting of
60 cm  60 cm  15 cm blocks which permits a
two-dimensional reconstruction of muon tracks
in two vertical, orthogonal planes. The verticalseparation of two layers of PRCs in the same plane
is 50 cm which allows the muon track direction to
be measured to an accuracy of 6 in the projected
plane.
To achieve an energy threshold of 1 GeV for
vertical muons, a total thickness 550 g cm2 in

6m
10 cm

10 cm
Fig. 1. A schematic diagram of 600 cm long PRC used as thebasic detector element in the GRAPES-3 tracking muon
detector.

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Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657

the form of concrete blocks is employed as anabsorber by placing a total of 15 layers of concrete
blocks above Layer-1. The robust structure of the
PRC permitted it, to support a huge load of 2.4 m
thick concrete absorber in a self-supportingmanner. The concrete blocks have been arrangedin the shape of an inverted pyramid to achieve anenergy threshold 1 (sec y) GeV for muons incident
on the detector with zenith angle y (with coverage
up to 45 ). A cross-section of two adjacent muon
detector modules is shown schematically in Fig. 2.
Four such modules, separated by a horizontal
distance of 130 cm at the base, constitute one
super-module.
Note that the top 7 layers of the concrete blocks
are shared as an absorber by all four modules in asuper-module. Therefore, the absorber above the10th layer from the base, is essentially a full-size
layer covering all four modules as seen in Fig. 2.
Finally, a 30 cm thick and 18 m  18 m concreteslab was cast as the top layer of the absorber inorder to make it weather-proof. Similarly at thebottom, a single 30 cm thick, 16 m  16 m concrete
block cast on the ground, serves as the ﬂoor for thesuper-module. This has been done for uniform
distribution of nearly 1200 t load of the absorberin a super-module to the soil below.
A picture of the PRCs and the concrete blocks
in a super-module is shown in Fig. 3. The PRCabsorber assembly of each super-module has been
enclosed within a large hall, with suitable doors
and windows for access and illumination, and tocontrol the humidity for safe operation of the
PRCs. Two heavy duty de-humidiﬁers are operated round the clock, inside each of the four halls
to maintain a low level of humidity (o50%), inview of nearly eight month long rainy season at
Ooty. Fig. 4 shows a picture of the four halls
housing the super-modules of the muon detector.
Some of the detectors of the GRAPES-3 shower
array [21] which have been placed on the roof andaround these halls are also visible.
2.1. Characteristic of PRCs
As described above, a muon detector is made up
of 600 cm long PRCs. The pipe used for making a
PRC, is closed at both ends with 6 mm thick, ﬂat
mild-steel plates equipped with hermetic, metal-toglass seals and a gas valve. After welding of theend plates, the ﬁnished cross-section of the PRC
had increased from 10 cm  10 cm to 10:2 cm 

Fig. 3. An inside view of a super-module showing the
proportional counters and layers of concrete blocks for twoof the four modules. The remaining two modules are locatedfurther inside.

45°
6m

2m
Layer 3Layer 2
Layer 1
Layer 0Fig. 2. Schematic of a muon detector module showing 4 layers of proportional counters embedded within concrete blocks.

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647

HV
PRC.

AMP.

DISC.
DWM

Fig. 5. A schematic diagram of PRC and associated electronics
used as the basic detector element of the tracking muon
detector.

Fig. 4. A view of a part of the GRAPES-3 array showing thefour halls housing the super-modules of the muon detector.

10:2 cm and therefore the 58 PRCs in a layer covera distance of 5.92 m. After the deployment it has
been found that the individual PRCs were aligned
to within 2 mm and with a cumulative offset of
t2 mm from their designated nominal positions.
The pipe also serves as the cathode of the PRC. A
100 mm diameter tungsten wire which also serves as
the anode, is mounted at the center of the pipeusing the hermetic seals.
After evacuation, each PRC is ﬁlled with P-10
(90% Ar and 10% CH4 ) gas above local atmospheric pressure and then sealed. A schematic viewof the PRC with its basic electronics, including acharge-sensitive pre-ampliﬁer and a discriminatorare shown in Fig. 5.
The amplitude distribution of the output pulses
from each PRC has been measured with a multichannel analyzer. A typical distribution, shown in
Fig. 6, displays a prominent peak due to cosmic
ray muons corresponding to an energy loss of20 keV in the 10 cm column of P-10 gas in the
PRC. The muon peak is well separated from theother two smaller peaks; the ﬁrst peak is due toﬂuorescent Ka X-ray emission in Fe at 6.4 keV andthe second minor peak at 8.6 keV is due to thesame effect in the Zn coating on the mild steel
pipe, used for its protection from rusting. Any
malfunction of the PRC is easily detectable from achange in the shape of this distribution andsuitable corrective steps can be initiated to restore
its performance.

Fig. 6. Pulse-amplitude distribution for a PRC in the muon
detector. The ﬁrst two minor peaks are due to ﬂuorescent Ka Xray emission in Fe at 6.4 keV (a), and in Zn at 8.6 keV (b). The
main peak at 20 keV (c) is due to the passage of cosmic raymuons.

The anode pulse from the PRC is shaped to an
exponential form with a decay time of 7 ms andampliﬁed ðgain ¼ 83Þ. The ampliﬁed pulse isdiscriminated at a threshold of 100 mV whichcorresponds to 0:2 of a minimum ionizing
particle. The output of the discriminator is a TTL
signal. The following information on the discriminator output pulse is recorded; (i) the arrival timewith a resolution of 167 ns and (ii) the width with aresolution of 333 ns. Due to exponential nature of
the ampliﬁed pulse, the pulse width is proportional
to the logarithm of the PRC pulse amplitude.
The number of particles passing through a PRC
in a given event are estimated as follows, assuming
T s to be the pulse-width for a minimum ionizing
particle, as calculated from the pulse-width monitoring system and t to be the decay time constant,the number of particles N in an event is given by
N ¼ eðT n T a Þ=t(1)

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where T n is the pulse width observed for the event.
This feature is used to estimate the total number ofmuons in large multiplicity events.

Each DWM card handles data from one layer
consisting of 58 PRCs. The schematic diagram of a
DWM card is shown in Fig. 8.
As explained in Section 2.1, the PRC output isshaped to an exponential form with a decay timeof 7 ms, thus the width of the discriminator output
pulse is proportional to the logarithm of PRC
pulse height. The DWM card continuously samples the PRC discriminator pulse, using a 6 MHz
clock and the address on RAM chip corresponding to the 167 ns interval when the PRC discriminator pulse is present is set to logical state ‘1’;otherwise in absence of the pulse, it is set to state‘0’. Use of 512 bytes on each RAM chip to store
the information on the PRC pulse, spans aduration of 85 ms with a time resolution of
167 ns. Therefore at any given instant of time,
information on the PRC pulse arrival time andwidth is available for the previous 85 ms on acontinuous basis. On the arrival of a trigger fromthe GRAPES-3 shower array (for details see [21])2.2. Signal processing & data acquisition system
An independent data acquisition system (DAQ)
is run at each of the four super-modules. All four
DAQs are simultaneously triggered when a
GRAPES-3 shower trigger is received [21]. The
DAQ consists of Digital Wave Memory (DWM)
cards, DWM control cards and the data acquisition PC (DAQ-PC) with all of the required
electronics designed and fabricated in-house.
Fig. 7 shows an overview of the DAQ system.
2.2.1. Digital wave memory card
As described earlier, the output pulse of thediscriminator contains information on the numberof muons and their arrival time in an individual
PRC. These pulses are sent to the DWM card.
DISC

Layer-OR

SHAPER
200 ns58 PRCs / Layer

Trigger from EAS Array
GPS 1 PPS Signal
Ether net

CRM data

Counting Rate
Monitor (CRM)
CRM control

PRC signal select

PRC signal for
Pulse Width Analysis

Veto for Rate Monitoring

Busy for EAS data taking

PRC Signal
Selector

DWM Card(1 Card / Layer)

Wave Memory Data

Wave Memory(RAM)

Timing &Width
Analysis
Time & Width

AMP

EAS Gate

PRC

DAQ control

648

DWM Control
Card(1 Card / Module)"MONITOR DATA""EAS Muon Data"

Pulse Width Analysis (PWA)&
Counting Rate Monitor (CRM)

Timing of signal&
Pulse Width
6 MHz Oscillator

Real-time ClockDAQ PC (1 PC / Supermodule)

Fig. 7. Overview of the DAQ system for muon detector. The DWM card stores the information on the pulses from a PRC for aduration of 85 ms. The data is then transferred to DWM control card and ﬁnally recorded in the hard disk in DAQ-PC.

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DWM Control
Card
Wave Memory Control

Digital Wave Memory Card

Wave Memory
Resolution = 167 ns
TTL
AMP DISC Signal
EAS Gate

649

Wave memory data Timing & Width
Analysis

58 Counter OR
SHAPER

Layer-OR

200 nsCounting Rate
Monitor58 PRCs
PRC signal select(From DAQ PC)

PRC signal selector for PWA
Signal for PWA(To DAQ PC)(From DAQ PC)
Busy for EAS data taking
1 Card / Layer

1 Card / Module

Fig. 8. Block diagram of the DWM card. Each card handles TTL signals from one layer of 58 PRCs. Also shown is the DWM control
card.

the DWM card freezes the contents of all RAM
chips, after a preset delay of 79 ms. Since at any
instant the DWM card holds information on PRC
pulses for the previous 85 ms, the preset delay of
79 ms allows data corresponding to 85279 ¼ 6 msprior to the arrival of the shower trigger to beaccessed. Due to exponential shaping of PRC
output pulse, the number of successive 167 ns
intervals with logic state ‘1’ provides direct
measure of the logarithm of pulse height. The
PRC signals are sampled for 6 ms before the arrivalof muons, since it takes 2 ms to generate the trigger
after the passage of a shower. The shower trigger
switches the DWM card to READ mode andstarts the transfer of data to the DWM control
card.
For further processing, each PRC pulse is reshaped into a TTL pulse of width 200 ns and an
OR of all re-shaped pulses in a layer, termed‘Layer-OR’, is generated. The performance of
PRCs in a layer is monitored by taking various

combinations of coincidences among the layers.
These coincidences between the Layer-OR signals
have an excellent sensitivity to the performance of
PRCs in a module.
2.2.2. DWM control card
Fig. 9 shows the block diagram of the DWM
control card. This card has been designed to takecare of the signals from the four DWM cards of amodule. The control card serves the following
functions after receiving a shower trigger:(1) Reads the stored signals, including the arrival
time and pulse-width information from DWM
cards for each layer.
(2) Monitors the rate of each Layer-OR and all
combinations of coincidences among LayerOR signals.
Observations have shown that most of the muons
unassociated with the recorded shower can be

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Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657
DWM Control Card

DWM Card

DAQ PC

Module SelectData SelectWave Memory

Wave Memory Control Module Control Gate
6 MHz Clock

Wave Memory Data

Busy for EAS data taking

EAS Gate

PRC 58 / Layer

4 Cards / Module

DAQ Control
Read / Write
6 MHz Clock

EAS
Trigger

Width / Timing Contol

Width / Timing
Analyzer

Width / Timing Data

EAS Data

Resolution:
Width = 333 ns
Timing = 167 ns

1 Card / Module

1 PC / Super-module

Fig. 9. Block diagram of the DWM control card which starts recording of PRC data, following the receipt of a trigger from the shower
array. Also shown are DWM card and DAQ-PC used for data recording.

identiﬁed and rejected, just with a strict cut on therecorded arrival time of muons, without taking
into account the arrival direction of the shower.
This reduces the probability of observing accidental muons (for a 3-layer hit) to t0:2 muons per
shower.
2.2.3. Data acquisition PC
A pair of I=O interface cards inserted in the
DAQ-PC handle all four DWM control cards in asuper-module. On the receipt of the shower trigger
from the GRAPES-3 array, the DAQ-PC sends acommand to freeze the contents of DWM cards
and subsequently transfers these data to the DWM
control card. The data in the DWM control card isthen written on a hard disk along with the timestamp having an accuracy of 1 ms. The time stamp
is generated from a real-time clock, running on agood quality, 10 MHz quartz oscillator. The realtime clock is synchronized using a GPS receiver
every second. The muon data and the air shower
data, which are recorded independently, arecollated during analysis by using the time stamps
recorded at the four muon super-modules and theshower array. A sample of the observed muontracks in a typical air shower is shown in Fig. 10.

The dead-time due to the data acquisition
process is about 5 ms per shower. Since the shower
trigger rate is 13 Hz, it contributes to a dead time
fraction of 7%. On an average around 400 bytes
are required for recording a shower for thetriggering conditions mentioned later. These data
are temporarily stored into a local hard disk in the
DAQ-PC for various checks and are subsequently
recorded on a CD or DVD for permanent storageand off-site analysis.
2.3. Monitoring of the performance of PRCs
The maintenance of the stability of gain anddetection efﬁciency of each PRC is an important
experimental requirement. In order to monitor the
performance of the muon detector, the pulse-widthof all 3712 PRCs is continuously measured with apulse width analyzer system. A typical pulse-width
distribution is shown in Fig. 11. Note that as aresult of exponential shaping of PRC pulse andsubsequent measurement of its width, the numberof ‘equivalent’ minimum ionizing particles entering a PRC are effectively digitized to an accuracyof 5% for 1–5 particles and 10% for more than
5 particles. The shape of the pulse-width distribution is utilized to detect noisy and/or leaky PRCs.

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50

57

10

20

30

40

50

57

Module11
0
L 3
L 1
L 2
L 0

0

57

50

40

30

20

10

0

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0

Module13
L 3
L 1
L 2
L 0

57

50

40

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10

0

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50

40

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0

0

10

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Module15
0
L 3
L 1
L 2
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0

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0

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0

Fig. 10. Muon tracks associated with a typical air shower. The PRCs hit are represented by ’ and straight lines joining these ’represent the muon tracks. A pair of lines, (L0–L2) and (L1–L3), represent two orthogonal views of each muon track. A total of 24
muons tracks could be reconstructed for this shower.

ARTICLE IN PRESS
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652

The 6.4 keV ﬂuorescent Ka X-ray emission peak in
Fe is used for calibration and monitoring of thegain of the PRCs. In addition, the counting rates
of any 3-layer and all 4-layer coincidences between
Layer-OR signals are also monitored continuously

to provide information on the health of the muon
detector.
In Fig. 12, a schematic diagram of the monitoring of the Layer-OR rate in DWM control card isshown. The multi-layer rate is very sensitive tosome of the atmospheric parameters, such as thebarometric pressure and the temperature proﬁle in
the upper atmosphere. The detected muon rate isalso affected by the inﬂuence of the inter-planetary
magnetic ﬁeld and the solar wind on the propagation of relatively lower energy ðt1011 eVÞ primarycosmic rays.
In Fig. 13, the variation in muon ﬂux throughN-layer coincidence rate (N ¼ 3 or 4), atmosphericpressure and temperature for a period of sevendays during 7–14 March 2001 are shown. The
temperature data shows a periodicity of one cycle
per day due to the solar heating. However, thepressure and muon rate data show a periodic
variation of 2 cycles per day. In Fig. 13, the 12 h
periodicity is seen in the muon rate from any
3-layers (a) in the top panel with an amplitude of0:3%, for all 4-layers, (b) in the second panel

7

Counting Rate (Hz)

6
5
4
3
2
1
0
0

5

10

15

20

25

30

35

40

Pulse Width (µs)
Fig. 11. A typical pulse width distribution for a PRC in themuon detector.

DWM Card
Signal from 58 PRCs

Shaper200 ns2.5 µsLayer-OR(58 PRCs)DAQ PCDWM Control Card

SHAPER

Combinationof Coincidences

Rate Monitor Gate

4-FoldAny-3

SHAPERSHAPER

3-Fold2-FoldLayer-ORMPX

Veto
For EAS Data-taking

Output 4-Fold
Output Any-3
Output Combination

Output Select

Module Address
Enable Timer
Read / Write

Timer control

Monitor data

Counting Rate
Monitor (CRM)
data

SHAPERCRM data select
PRC signal
Selector

Control for
PRC signal
Selector

Control for Pulse Width Analysis

Signalfor Pulse Width
Analysis (PWA)

PRC Signal for Pulse Width Analysis

4 Card / Module

1 Card / Module

Pulse Width
Analysis
Circuit1 PC / Supermodule

Fig. 12. Block diagram of monitoring system in the DWM control card. Also shown are DWM card and the DAQ-PC.

ARTICLE IN PRESS
Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657

653

(a)

(b)

(c)

(d)
(e)

Fig. 13. Observed variations over a seven day period during 7–14 March, 2001; (a) any 3-layer coincidence rate in Hz, (b) 4-layer
coincidence rate in Hz, (c) atmospheric pressure in hPa, (d) temperature inside a muon hall and (e) temperature outside muon halls.

with same amplitude ð0:3%Þ and in atmosphericpressure, (c) in the third panel with an amplitudeof 0:2%. The last panel in Fig. 13 shows that thetemperature inside a muon hall, (d) remains almost
constant to within 0:2  C, however the temperature outside muon halls, and (e) shows signiﬁcant variation ð8222  CÞ dependent on the local
weather.
The periodic variation of two cycles per day inthe pressure data is a well-known phenomenon at
low latitudes. An anti-correlation between thepressure wave and the muon counting rates isclearly visible in Fig. 13. This anti-correlation canbe explained in terms of a change in the atmospheric absorption of muons due to the pressure
wave. These observations are of critical importance while analyzing data, for studying varioustransient phenomena, including those due tocosmic, solar [20] or atmospheric origin. The
pressure effect is also clearly seen in the shower
counting rate [21].

Due to its large area, the GRAPES-3 muon
detector rate, after correction for the changes in
the atmospheric pressure, has been observed to benearly constant with an rms variation of 0:1%.
This enables the GRAPES-3 muon detector to be avery sensitive monitor of the Solar activity,
particularly, for observations on phenomena suchas Ground Level Enhancements (GLEs) and
Forbush decreases. Fig. 14 shows a remarkabledecrease of almost 10% in the cosmic ray muon
ﬂux, on the morning of 29 October, 2003,  20 hafter a large X17 class solar ﬂare on 28 October,2003. In fact the muon ﬂux had started to decrease
from 22 October, 2003, almost six days prior to theonset of the large Solar ﬂare at 09:51 Hrs UTC on
28 October, 2003, indicated by the ﬁrst ð.Þ marker
in Fig. 14, and before the arrival of the coronalmass ejection (CME) at the Earth at 06:13 Hrs
UTC, on 29 October, 2003 as indicated by thesecond ð.Þ marker in the same ﬁgure. The muon
ﬂux continued to display large variation over thisARTICLE IN PRESS
Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657

654

2%

Relative Intensity

0%
-2%
-4%
-6%
-8%

Solar
flare CME

-10%
19 20 21 22 23 24 25 26 27 28 29 30 31 01 02
Fig. 14. Variation in the muon (4-layer) rate over a two week
period in October–November 2003 overlapping occurrence of a
X17 class Solar ﬂare at 9:51 Hrs UTC on 28 October indicatedby ﬁrst marker and arrival of CME at the Earth at 6:13 Hrs
UTC on 29 October indicated by second marker.

six day period along with a systematic decrease
2% of the quiescent ﬂux. The observed muon
ﬂux continued its exponential recovery beyond 2
November, 2003, reaching its pre-Forbush decrease level by 10 November, 2003. In fact, a lot
more instrumentation has been added to the
GRAPES-3 muon detector to enable detailed
studies on cosmic ray ﬂux variations related tothe solar phenomenon whose details are discussed
elsewhere [20,26,27].

3. Observations and expectations
Due to sensitivity of the PRCs to low energy grays from radioactivity in the concrete absorber,the counting rate of individual PRCs is quite large,
200 Hz. Also, the PRC is a relatively slow
detector with a pulse rise time 500 ns. The decay
time of the pulse has been adjusted to be 7 ms toenable an accurate measurement of pulse amplitude through digitization of the pulse-width. Dueto these two factors, there is a small but nonnegligible probability that a few PRCs are ﬁred
randomly with each shower trigger. However, theaccidental muon rate is reduced to t0:2 pershower by a cut of 3 ms relative to the shower
trigger, on the time of arrival of muons.

In addition, a comparison of the direction of amuon track with that of shower helps in further
reducing the number of accidental muons. The
angular resolution of the muon detector is 6 ineach of the two projected planes. It has been
observed that by putting the requirement that thedirection of the re-constructed muon track shouldmatch the projection of the arrival direction of theshower to within 10 , the number of PRCs
triggered by muons, unrelated to the shower isreduced to t0:07 per shower [28].
The estimate of the number of muon tracks ineach module is quite accurate for a small numberof muons. The layer efﬁciency has been independently estimated and is found to be 498%. Afterallowing for the small gaps between the PRCs, themissing probability is estimated to be o0:3% for
single muon tracks in a module. Based on the layer
efﬁciency of 498%, the muon detection efﬁciency
is estimated to be 492% for all 4-layer and499:8% for any 3-layer coincidences, respectively.
However, the number of muons is generally underestimated if the number incident on a module islarge. This is due to the fact that two muons
incident on the adjacent PRCs are likely to be
counted as one, particularly at larger zenith angles.
Using a simple track-ﬁnding algorithm and random distribution of muons, simulations haveshown that the under-estimate becomes as large
as 20% when the number of muons exceeds 10 in asingle module. However, this factor is taken into
consideration during data analysis through a
response function obtained from simulations.

4. Simulations of muon multiplicity distribution
It is well-known that the protons constitute thelargest fraction of primary particles for showers
which are selected using a trigger based on densityof particles which are predominantly electrons
near the core. While the electron component of ashower, as observed in the lower atmosphere, isrelatively insensitive to the atomic number ofthe primary cosmic ray particle, the muon multiplicity is one of the most sensitive parameters
for determining the atomic number of the primaryparticle initiating the shower. Therefore it isARTICLE IN PRESS
Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657

important to study muon multiplicity distributions
for various triggering conditions used for selecting
showers.
Fig. 15 shows the muon multiplicity distribution
for showers selected with 3-line coincidence trigger
which also requires the triggering of an additional
7 detectors from those located within the inner 127
detectors of the array [21]. In a large sample ofshowers, the total number of detected particles is
proportional to the total number of particles in theshower at the observational level (shower size),
which in turn is proportional to the energy of the
primary particle. Thus the total number ofdetected particles represents, the primary energy
on a statistical basis. Thus a further cut on
the total number of detected particles Sni , of
50pSni p100 is also imposed. The expected muon
multiplicity distributions were generated fromMonte Carlo simulations using the CORSIKA
code [29] and QGSJet model [30] for hadronicinteractions. For a comparison, the muon multiplicity distributions for g-ray, proton and Fe
nitiated showers are also shown in Fig. 15. Theenergy of g-ray, proton and Fe primary weregenerated by assuming that the ﬂux in each case isrepresented by the same differential power law
spectrum ðE 2:7 Þ. For each of the distributions in

Fraction of Showers

1

10-1

10-2

655

Fig. 15, the total number of showers has beennormalized to be 1.
It is interesting to note that the experimentallyobserved distribution (a) in Fig. 15 lies between the
expected distributions for proton (c) and iron(d) primaries. Clearly, the observed distribution
shown in Fig. 15 requires a signiﬁcant contributionof heavier nuclei. This discrimination based on theatomic number is made possible because theshowers initiated by heavier nuclei such as irongenerate a much larger number of muons ascompared to lighter nuclei such as proton and
He. It has also been shown through simulations[17] that larger muon multiplicities are predominantly due to the contribution of medium heavy
and heavy nuclei in the primary ﬂux, when
showers are selected through a well-deﬁned
criterion for the shower particles, such as particle-sum, etc. It is this sensitivity of different
multiplicity regions to different nuclear groups
which is expected to enable a determination of therelative contribution of each nuclear group to theprimary cosmic ray ﬂux.
A use of the muon-poor criterion for showers is
expected to reject hadron induced showers efﬁciently when data from a large muon detector isused. From the g-ray simulations shown in Fig. 15,
it is estimated that \90% of hadron induced
showers can be rejected with a selection of showers
where no muon is detected, while only a few
percent of g-ray induced showers get rejected withthis cut. This provides a powerful tool while
searching for cosmic sources of UHE g-rays.

5. Summary
b

10-3

c

a

20

30

d

10-4
0

10

40

50

Number of Muons Detected
Fig. 15. (a) The observed muon multiplicity distribution forshowers is shown using n with a cut on the ‘particle-sum’
Sni ; 50pSni p100. Monte Carlo simulations with the same cut
are shown, (b) g-ray primaries using m, (c) proton primaries
using and (d) Fe primaries using ’.

In this paper, detailed features of the GRAPES3 muon detector have been presented. The area of
560 m2 makes it one of the largest tracking muon
detectors in the world. The unique features of thedetector electronics makes it possible to measure
the pulse arrival time and width for each PRC
where a muon has traversed. The arrival time ofeach muon, measured to an accuracy of 167 ns,
makes it possible to reject most of the muons
which are physically unassociated with a shower.
The angular resolution of the muon detector is

ARTICLE IN PRESS
656

Y. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 643–657

6 in each of the two projections. A comparisonof the muon direction with that of the shower
in two orthogonal projections, further reduces
the background of unassociated muons to anegligible level.
In cases where more than one muon has passed
through a PRC, the pulse-width is used to estimate
the total number of muons. Using an efﬁcient
track reconstruction algorithm, the total numberof X1 GeV muons incident on the detector can be
estimated to a reasonable accuracy up to a
maximum of 160 muons. Using muon multiplicity
distributions, it is expected that the mass composition of cosmic rays can be measured accurately inthe energy range from 3  1013 23  1016 eV. A cut
on the muon content in a shower permits an
efﬁcient rejection ð\90%Þ of hadron induced
showers and a preferential selection of g-ray
induced showers, during the search for cosmic gray sources above 1013 eV.

Acknowledgements
We thank D.B. Arjunan, K. Manjunath, B.
Rajesh and C. Ravindran for their help in testing,installation and operation of the proportionalcounters and the electronic modules. The administrative services of V. Viswanathan during allphases of the experiment are gratefully acknowledged. It is a pleasure to thank A.A. Basha, G.P.
Francis, I.M. Haroon, V. Jeyakumar, A. Pushpanathan and K. Ramadass for their help in the
fabrication, assembly and installation of various
mechanical components and detectors. We also
thank M.A. Ali, P. Amalraj, M. Iyer, B. Jasper,
G.C. Kamat, A. Pratheep, T.T. Sreedevi and S.
Thirunavukkarasu for their help during certain
phases of the experiment. The Japanese membersof the GRAPES-3 collaboration acknowledge thepartial ﬁnancial support from the Ministry of
Education and Science of the Government ofJapan for the experiment. The GRAPES-3 collaboration thanks N.K. Mondal, V.S. Narasimham
and their colleagues in the TIFR-OCU Proton
Decay Collaboration for the loan of the proportional counters which have been used in themuon detector.

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